The locations of underground utilities such as power, water, sanitary, telephone, gas, cable television (CATV), and non-potable may be marked with devices known as passive underground electronic markers. Typically, each marker contains one or more Inductor-Capacitor (LC) circuits, or equivalent circuitry, each of which is tuned to a unique frequency associated with a particular utility type. In that regard, each circuit typically includes a coil of wire that responds to reception of a signal transmitted at a particular frequency. The markers are buried in a trench with the utility structure to be located during surveying, maintenance operations, or other activities requiring precise location of the utility.
In order to locate a buried marker, an operator moves across the surface of the ground with a locator, which typically includes both a transmitter and a receiver. The transmitter sends out an electromagnetic signal tuned to the unique frequency of the marker associated with a particular utility type. Upon receiving the transmitted signal, the marker resonates and thus produces an electromagnetic response to the transmitted signal. This response is received by the receiver and converted to a signal that is detectable by the operator. The operator may mark the pinpointed location on the ground using chalk, paint, or other surface marking indicia, and then move on to find the next marker buried along the utility structure. Examples of passive marker and locator systems are disclosed U.S. Pat. Nos. 5,699,048, 6,097,293, 6,380,857 and 6,388,575, the disclosures of which are incorporated herein by reference.
The locator sends out a specific frequency that is the same frequency as the marker sought to be identified. Electronic markers sensitive to the transmitted frequency will be stimulated. A common problem associated with conventional utility marker location technologies arises due to the fact that markers associated with other utility types (i.e., different from the type sought to be identified) may be stimulated and respond to the transmitted frequency as well. Current marker locators receive signals back from any marker that has been stimulated. The response from those markers not at the transmit frequency have a detectable signature or response characteristic. The amplitude of this response generally decreases as the transmit frequency gets further from the resonant frequency of the marker.
Problematically, current locators cannot determine that the response received at a particular transmit frequency is from a different marker type from the marker type intended to be detected. In that regard, the response from a particular marker decreases as the distance between the marker and the locator increases; mathematically, the response can be expressed as the function 1/r^4, where r is the distance between the marker and locator. In accordance with the 1/r^4 response signal loss, a marker at the wrong frequency, but at a distance of only six inches from the locator, can have a greater response at the detection frequency than the correct frequency marker at a distance of five feet.